The OVERALL OBJECTIVE of the proposed investigation is to elucidate the roles of the interferon-inducible RNA-regulated protein kinase (PKR) in the actions that interferons (IFN) mediate on viral and host functions. The three SPECIFIC AIMS of our proposed continuation studies are as follows: (1) To further delineate the roles of the PKR protein in the host responses to virus infection. To characterize the function of the PKR protein in human cells in culture, through the use of established HeLa, U, and Huh cell clones that are stably deficient in PKR because of targeted gene silencing compared to appropriate knockdown-control cell clones. These human cell clones will be examined for the mechanisms by which PKR affects virus replication, with emphasis on vaccinia virus (wild-type and E3L mutant), and measles virus (wild- type, V and C mutants) and adenovirus (VAI mutant), viruses with human cell tropism, and for the cellular apoptotic and IFN inducing phenotypes in response to different stimuli of the knockdown compared to control cell clones. (2) To elucidate the biochemical mechanism by which the PKR protein confers the PKR-dependent biological phenotypes characterized under aim 1. To attempt to determine the biochemical functions of the PKR protein (RNA binding activity; catalytic activity; PKR domain) necessary to complement the biological phenotypes (including apoptosis, virus growth and IFN production) and biochemical changes characteristic of the human PKR knockdown cells, using wild-type and mutant forms of cDNA expression constructs for the human PKR protein engineered to circumvent knockdown, and the mouse PKR and fish PKZ proteins. (3) To further delineate the structure of the 5'-flanking region of the Pkr gene and identity of trans-acting factors required for interferon-inducible as well as basal transcriptional activity. To map the major Pkr transcription sites, using RNA from uninfected and infected and untreated and IFN-treated cells and tissues, and to elucidate the basis of the tissue-specific differences in Pkr mRNA size multiplicity. To delineate the importance of Sp1 and Sp3 protein binding to the novel 15-bp DNA element and upstream sites in IFN inducible as well as basal transcriptional activity.